Friday, April 14, 2006

Calibrating the charging voltage

We calibated the charging voltage at increments of 10kV, 15kV, 20kV and 25kV using a Fluke high-voltage probe connected to the Thyratron capacitor bank and read with a Fluke digital meter. The results of these measurements indicated that the system charged to about 19kV when the control panle meter actually readout a voltage 0f 24.5kV. This error is merely caused by an error in the high voltage divider at the output of the transformer that needs to be fixed. Unfortunately, this also meant that the maximum voltge that the system could be charged to was 19kV.

During these measurement, it was noticed that erratic charging occured at the higher voltages above 15kV and, in fact, the sytem operated correctly only every other pulse. The pulse rate was then slowed down to one pulse every 5 seconds. Although the initial timing adjustment seemed logical and worked up to 15kV, further insight from the above trace indicated that it is desirable that there be a longer recovery period after thyratron firing before commanding the system to charge. The Thyratron should recover very rapidly (less than 1ms)but the induced transient noise in the system may need a much longer recovery period in order for the present firing angle electronics to work properly. This can be seen in the slight negative signal right after Thyratron firing and to last about 0.2 seconds. Rather than increasing the Charging Delay to about 2.5 seconds as had been done, the delay should have been reduced to about 0.8 seconds. This would have resulted in a charging delay of 0.8 seconds, a charge time of 0.9 seconds, and a charge hold time of 0.8 seconds for a total cycle time of 2.5 seconds. As mentioned previously, the system was finally run at 0.4Hz at 15kV for about one hour with no adverse heating of the charging resistors being noticed. Although the evidence presented somewhat confirms that timing has been the main problem during the past 4 months, only operation at higher voltages and longer periods of time can guarantee that the charging resistor problem is “really” fixed. (from Randy's report)

The below is the experimental data.

DIAL

H.V.Probe(kV)

Control PanelMeter (kV)

C-2153J1 (V)

6.16

11.10

14.0

3.35

7.10

13.00

16.5

3.91

8.72

15.25

19.6

4.6

10.00

19.00

24.5

5.7

Randy put a Rogowski coil to measure the current pass from the Thyratron to ground. When we carefully adjusted the time delay bewteen the rail gap and the Thyratron firing, we certainly found the current closed to 0. This result totally matched Randy's simulation.